U.S. patent application number 13/320156 was filed with the patent office on 2012-03-08 for copper zinc tin chalcogenide nanoparticles.
This patent application is currently assigned to E.I. Du Pont De Nemours and Company. Invention is credited to Jonathan V. Caspar, Lynda Kaye Johnson, Meijun Lu, Irina Malajovich, Daniela Rodica Radu, H. David Rosenfeld.
Application Number | 20120055554 13/320156 |
Document ID | / |
Family ID | 45769783 |
Filed Date | 2012-03-08 |
United States Patent
Application |
20120055554 |
Kind Code |
A1 |
Radu; Daniela Rodica ; et
al. |
March 8, 2012 |
COPPER ZINC TIN CHALCOGENIDE NANOPARTICLES
Abstract
This invention relates to nanoparticles of kesterite (copper
zinc tin sulfide) and copper zinc tin selenide nanoparticles, inks
and devices thereof, and processes to prepare same. The
nano-particles are useful to for the absorber layer as a p-type
semiconductor in a thin film solar cell application.
Inventors: |
Radu; Daniela Rodica; (West
Grove, PA) ; Caspar; Jonathan V.; (Wilmington,
DE) ; Johnson; Lynda Kaye; (Wilmington, DE) ;
Rosenfeld; H. David; (Drumore, PA) ; Malajovich;
Irina; (Swarthmore, PA) ; Lu; Meijun;
(Hockessin, DE) |
Assignee: |
E.I. Du Pont De Nemours and
Company
Wilmington
DE
|
Family ID: |
45769783 |
Appl. No.: |
13/320156 |
Filed: |
May 21, 2010 |
PCT Filed: |
May 21, 2010 |
PCT NO: |
PCT/US10/35734 |
371 Date: |
November 11, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61180179 |
May 21, 2009 |
|
|
|
61180184 |
May 21, 2009 |
|
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Current U.S.
Class: |
136/264 ;
106/31.92; 252/519.14; 427/372.2; 977/773 |
Current CPC
Class: |
C09D 11/03 20130101;
H01L 31/0327 20130101; C09D 11/52 20130101; H01L 31/206 20130101;
Y02P 70/50 20151101; B82Y 30/00 20130101; Y02E 10/50 20130101; Y02P
70/521 20151101 |
Class at
Publication: |
136/264 ;
252/519.14; 106/31.92; 427/372.2; 977/773 |
International
Class: |
H01L 31/0264 20060101
H01L031/0264; C09D 11/00 20060101 C09D011/00; B05D 3/02 20060101
B05D003/02; B05D 7/24 20060101 B05D007/24; H01B 1/02 20060101
H01B001/02; B05D 3/00 20060101 B05D003/00 |
Claims
1. A quaternary nanoparticle comprising copper, zinc, tin, a
chalcogen, and a capping agent, wherein the chalcogen is selected
from the group consisting of sulfur, selenium and mixtures
thereof.
2. A nanoparticle according to claim 1 that has a longest dimension
of about 1 nm to about 1000 nm, and/or a kesterite structure.
3. A nanoparticle according to claim 1 wherein the molar ratio of
copper to zinc to tin to chalcogen is about 2:1:1:4; or the molar
ratio of copper to zinc plus tin is less than one; or the molar
ratio of zinc to tin is greater than one.
4. A nanoparticle according to claim 1 wherein the capping agent
comprises (a) an organic molecule that comprises a nitrogen-,
oxygen-, sulfur-, or phosphorus-based functional group; (b) a Lewis
base; or (c) an electron pair-donor group, or a group that can be
converted into an electron pair-donor group, that has a boiling
point of less than about 150.degree. C. at ambient pressure.
5. A composition comprising a plurality of nanoparticles according
to claim 1, wherein the composition has a particle size
distribution such that the average longest particle dimension is in
the range of about 10 nm to about 100 nm with a standard deviation
of about 10 nm or less.
6. A process for preparing a copper-zinc-tin-chalcogenide
quaternary nanoparticle, comprising (a) forming in a solvent a
reaction mixture of (i) metal salts and/or complexes of copper,
zinc and tin, (ii) one or more chalcogen precursor(s), and (iii) a
first capping agent, and (b) heating the reaction mixture to form a
nanoparticle.
7. A process according to claim 6 wherein individual metal salts
and/or complexes of copper, zinc and tin are separately added in
sequence to a mixture of a solvent and the first capping agent to
form a reaction mixture, followed by the addition to the reaction
mixture of a chalogen precursor.
8. A process according to claim 6 comprising (a) contacting the
reaction mixture with a second capping agent that has greater
volatility than the first capping agent to exchange in the
nanoparticle the second capping agent for the first capping agent;
or (b) recovering the nanoparticle from the reaction mixture
followed by contacting the nanoparticle with a second capping agent
that has greater volatility than the first capping agent to
exchange in the nanoparticle the second capping agent for the first
capping agent.
9. A process according to claim 8 wherein the second capping agent
has a boiling point of less than about 200.degree. C. at ambient
pressure.
10. An ink comprising an organic solvent and a composition
comprised of a plurality of nanoparticles according to claim 1.
11. An ink according to claim 10 further comprising one or more
binders or surfactants selected from the group consisting of
decomposable binders, decomposable surfactants, cleavable
surfactants, surfactants with a boiling point less than about
250.degree. C., and mixtures thereof.
12. A composition comprising a plurality of nanoparticles according
to claim 1 fabricated as a film.
13. A method of forming a film comprising depositing on a substrate
a layer of a composition that comprises a plurality of
nanoparticles according to claim 1, and drying the deposited layer
of composition to remove solvent therefrom.
14. A method according to claim 13 further comprising heating the
film in an atmosphere to anneal it; wherein the atmosphere is
inert, or comprises a reactive component selected from the group
consisting of selenium vapor, sulfur vapor, hydrogen, hydrogen
sulfide, hydrogen selenide, and mixtures thereof.
15. A method according to claim 13 wherein the film comprises a
first capping agent, and the method further comprises contacting
the film with a second capping agent that has greater volatility
than the first capping agent to exchange in the nanoparticles of
the film the second capping agent for the first capping agent.
16. A method according to claim 15 wherein the second capping agent
has a boiling point of less than about 200.degree. C. at ambient
pressure.
17. An electronic device comprising a film that comprises a
plurality of nanoparticles according to claim 1.
18. A device according to claim 17 wherein the film comprises
multiple layers; and a first layer comprises a plurality of
nanoparticles according to claim 1, and a second layer comprises a
binary semiconductor, a chalcogen source, a sodium-containing
material, or a mixtures thereof.
19. A device according to claim 18 wherein a chalcogen source is
selected from the group consisting of chalcogen particles, binary
chalcogenide particles, and mixtures thereof; and/or a
sodium-containing material is selected from the group consisting of
sodium salts of deprotonated alcohols, sodium salts of deprotonated
acids, sodium hydroxide, sodium acetate, sodium sulfide, and
mixtures thereof.
20. A device according to claim 18 wherein the first layer is
adjacent to the second layer.
Description
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) from, and claims the benefit of, the following U.S.
Provisional Applications: No. 61/180,179, No. 61/180,181, No.
61/180,184, and No. 61/180,186; each of which was filed on May 21,
2009, and each of which is by this reference incorporated in its
entirety as a part hereof for all purposes.
TECHNICAL FIELD
[0002] This invention relates to copper zinc tin chalcogenide
nanoparticles, compositions thereof, and their use in thin films
and devices. The nanoparticles are a p-type semiconductor useful
for the absorber layer in a solar cell.
BACKGROUND
[0003] Solar cells, also termed photovoltaic or PV cells, and solar
modules convert sunlight into electricity. These devices utilize
the specific electronic properties of semiconductors to convert the
visible and near visible light energy of the sun into usable
electrical energy. This conversion results from the absorption of
radiant energy in the semiconductor materials, which frees some
valence electrons, thereby generating electron-hole pairs. The
terms "band gap energy", "optical band gap" and "band gap" as used
in the art refer to the energy required to generate electron-hole
pairs in a semiconductor material, which in general is the minimum
energy needed to excite an electron from the valence band to the
conduction band.
[0004] Solar cells have been traditionally fabricated using silicon
(Si) as a light-absorbing, semiconducting material in a relatively
expensive production process. To make solar cells more economically
viable, solar cell device architectures have been developed that
can inexpensively make use of thin-film, light-absorbing
semiconductor materials such as
copper-indium-gallium-sulfo-di-selenide, Cu(In, Ga)(S, Se).sub.2,
also termed CIGS. This class of solar cells typically has a p-type
absorber layer sandwiched between a back electrode layer and an
n-type junction partner layer. The back electrode layer is often
Mo, while the junction partner is often CdS. A transparent
conductive oxide (TCO) such as but not limited to zinc oxide doped
with aluminum is formed on the junction partner layer and is
typically used as a transparent electrode. CIS-based solar cells
have been demonstrated to have power conversion efficiencies
exceeding 19%.
[0005] The development of nanocrystalline CIGS and the production
of films therefrom have been reported in a number of papers and
patent applications. For example, WO 08/021604 describes the
production of chalcopyrite nanoparticles by reacting a metal
component with an elemental chalcogenide precursor in the presence
of an alkylamine solvent with a normal boiling temperature of above
about 200.degree. C. and an average particle size of from about 5
nm to about 1000 nm. Reaction temperatures were varied from
265-280.degree. C. WO 99/037832 discloses a method of forming a
film of metal chalcogenide semiconductor material on a substrate
from a colloidal suspension comprising metal chalogenide
nanoparticles and a volatile capping agent. The colloidal
suspension is made by reacting a metal salt with a chalcogenide
salt to precipitate a metal chalcogenide, recovering the metal
chalcogenide, and admixing the metal chalcogenide with a volatile
capping agent, while WO 09/051862 discloses semiconductor thin
films formed from nonspherical particles, including inks
thereof.
[0006] Despite the demonstrated potential of CIGS in thin-film
solar cells, the toxicity and low abundance of indium and selenium
are major impediments to the widespread use and acceptance of CIGS
in commercial devices. An alternative for absorber layers of thin
film solar cells is copper zinc tin sulfide, Cu.sub.2ZnSnS.sub.4
(CZTS). It has a direct bandgap of about 1.5 eV and an absorption
coefficient greater than 10.sup.4 cm.sup.-1. In addition, CZTS does
not include any toxic or nonabundant elements. Whereas crystals of
CIGS have a chalcopyrite structure, CZTS crystals pack in a
kesterite structure that is related to the chalcopyrite structure
by doubling along the c-axis.
[0007] Currently, the development of solar cells based upon CZTS
lags significantly behind CIGS-based solar cells. Although the
first CZTS heterojunction PV cell was reported in 1996, the current
record efficiency for a CZTS cell is 9.6%. To date, thin films of
CZTS have been prepared via sputtering of Cu, SnS, and ZnS
precursors, hybrid sputtering, pulsed laser deposition, spray
pyrolysis of halides and thiourea complexes,
electrodeposition/thermal sulfurization, E-beam Cu/Zn/Sn/thermal
sulfurization, sol-gel followed by thermal sulfurization and via
printed precursors. A hybrid solution-particle approach to CZTS
involving the preparation of a hydrazine-based slurry comprising
dissolved Cu--Sn chalcogenides (S or S--Se), Zn-chalcogenide
particles, and excess chalcogen has been reported. Hydrazine is a
highly reactive and potentially explosive solvent that is described
in the Merck Index as a "violent poison".
[0008] A need thus remains for the development of improved CZTS
particles, and compositions, films and devices made therefrom.
SUMMARY
[0009] In one embodiment, there is provided in this invention a
quaternary nanoparticle that contains copper, zinc, tin, a
chalcogen, and a capping agent, wherein the chalcogen is selected
from the group consisting of sulfur, selenium and mixtures
thereof.
[0010] In another embodiment, there is provided in this invention a
composition that contains a plurality of quaternary nanoparticles
that contains copper, zinc, tin, a chalcogen, and a capping agent,
wherein the chalcogen is selected from the group consisting of
sulfur, selenium and mixtures thereof.
[0011] In a further embodiment, there is provided in this invention
an ink that contains an organic solvent, and a composition that
contains a plurality of quaternary nanoparticles that contains
copper, zinc, tin, a chalcogen, and a capping agent, wherein the
chalcogen is selected from the group consisting of sulfur, selenium
and mixtures thereof.
[0012] In yet another embodiment, there is provided in this
invention a film fabricated from a composition that contains a
plurality of quaternary nanoparticles that contains copper, zinc,
tin, a chalcogen, and a capping agent, wherein the chalcogen is
selected from the group consisting of sulfur, selenium and mixtures
thereof. In yet another embodiment, there is provided in this
invention an electronic device that contains the above described
film.
[0013] In yet another embodiment, there is provided in this
invention a process for preparing a copper-zinc-tin-chalcogenide
quaternary nanoparticle, by (a) forming in a solvent a reaction
mixture of (i) metal salts and/or complexes of copper, zinc and
tin, (ii) one or more chalcogen precursor(s), and (iii) a capping
agent, and (b) heating the reaction mixture to form a
nanoparticle.
[0014] In yet another embodiment, there is provided in this
invention a method of forming a film by depositing on a substrate a
layer of a composition that contains a plurality of quaternary
nanoparticles that contains copper, zinc, tin, a chalcogen, and a
capping agent, wherein the chalcogen is selected from the group
consisting of sulfur, selenium and mixtures thereof; and drying the
deposited layer of composition to remove solvent therefrom.
[0015] This invention addresses a need for the development of
environmentally sustainable nanocrystalline semiconductors and inks
of such semiconductors, based upon elements of high natural
abundance and low toxicity. Furthermore, there is a need for thin
films and devices based upon such materials. In light of declining
supplies of fossil fuels and the growing global energy demands,
particularly desirable are nanocrystalline, environmentally
sustainable semiconductors useful in the production of thin film
absorber layers suitable for use in solar cells. Also particularly
desirable are nanocrystalline materials that can be annealed at
lower temperatures. During nanoparticle synthesis, moderate
reaction temperatures, short reaction times, and minimal steps in
the isolation and purification process are desirable to lower cost,
conserve energy and develop a commercially feasible process.
Particularly desirable is a process of nanoparticle synthesis in
which the as-synthesized reaction mixture utilizes solvents and
reagents with relatively low toxicity and may serve as the ink or
ink precursor, with no isolation or purification steps.
[0016] Moreover, nanocrystalline semiconductors, and synthetic
routes to such nanoparticles, are of interest because such
nano-sized particles possess a number of unique physiochemical
properties such as quantum size effects, size-dependent chemical
reactivity, optical non-linearity, and efficient photoelectron
emission. Films of nanocrystalline semiconductors can serve as
precursor films to the absorber layer in thin-film solar cells with
significantly lower annealing temperatures than films prepared from
larger particles. Beyond the inherent energy savings associated
with a reduced thermal budget, the reduced deposition temperature
allows the use of lower cost substrates such as soda-lime glass and
potentially even polymer-based substrates, while alleviating
substrate out diffusion and relieving thermal stress.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Various features and/or embodiments of this invention are
illustrated in drawings as described below. These features and/or
embodiments are representative only, and the selection of these
features and/or embodiments for inclusion in the drawings should
not be interpreted as an indication that subject matter not
included in the drawings is not suitable for practicing the
invention, or that subject matter not included in the drawings is
excluded from the scope of the appended claims and equivalents
thereof.
[0018] FIGS. 1-1, 1-2, 1-3 and 1-4 show transmission electron
micrographs ("TEM") of nanoparticles prepared in Example 1.
[0019] FIG. 2 shows a TEM of nanoparticles prepared in Example
2.
[0020] FIG. 3 shows a perspective view of a stack of layers in a
solar cell.
[0021] FIG. 4 shows performance data for a film as prepared in
Example 15.
DETAILED DESCRIPTION
[0022] This invention involves quaternary nanoparticles that
contain copper, zinc, tin and sulfur and/or selenium, and are
referred to in certain embodiments by the term "CZTS", which can be
a compound such as Cu.sub.2ZnSnS.sub.4. Examples of other compounds
included herein are"CZTSe", which refers to Cu.sub.2ZnSnSe.sub.4;
and "CZTS/Se", which encompasses all possible combinations of
Cu.sub.2ZnSn(S, Se).sub.4, including Cu.sub.2ZnSnS.sub.4,
Cu.sub.2ZnSnSe.sub.4 and Cu.sub.2ZnSnS.sub.xSe.sub.4, where
0.ltoreq.x.ltoreq.4. The terms "CZTS", "CZTSe" and "CZTS/Se" thus
further encompass copper zinc tin sulfide/selenide semiconductors
with a range of stoichiometries, for example those described by the
formla Cu.sub.1.94Zn.sub.0.63Sn.sub.1.3S.sub.4. That is, the
stoichiometry of the elements is not limited strictly to a ratio of
2Cu:1Zn:1Sn:4S/Se (i.e. where the molar ratio of copper to zinc to
tin to chalcogen is about 2:1:1:4). In other embodiments, the molar
ratio of Cu/(Zn+Sn) can be less than 1.0, or the molar ratio of
zinc to tin of can be greater than one. In copper-poor CZTS solar
cells, for example, the molar ratio of Cu/(Zn+Sn) is less than 1.0
although, for high efficiency devices, a molar ratio of zinc to tin
of greater than one is frequently desirable. The compounds may also
be doped with small amounts of various dopants selected from the
group consisting of binary semiconductors, elemental chalcogens,
sodium, and mixtures thereof.
[0023] CZTS crystallizes with, and thus has, a kesterite structure,
where the term "kesterite" refers to materials belonging to the
kesterite and stannite family of minerals. Kesterite is a mineral
name referring to crystalline compounds in either the I4- or I4-2m
space groups having the nominal formula Cu.sub.2ZnSnS.sub.4. Other
metals, such as Fe, often substitute for Zn, however. When a
majority of the Zn has been substituted by Fe it is referred to as
stannite. Kesterite was once thought to exist only in the I4-form
with Cu on positions 2a (0,0,0) and 2c (0,1/2,1/4), and Zn on 2d
(1/2,0,1/4), however it has been shown to exhibit disorder in which
Zn may occupy Cu sites and vice versa, lower the symmetry to I4-2m.
The difference in cation ordering is indistinguishable by x-ray
diffraction, owing to the similarity of x-ray scattering factors
for Cu and Zn. It can be distinguished by neutron diffraction.
Materials produced by this route yield diffraction patterns
containing peaks consistent with the kesterite structure and
distinct from the monosulfides and oxides. X-ray absorption
spectroscopy (XAS) likewise reveals spectral features unique to the
kesterite form and distinct from the monosulfides and oxides. In
the case of XAS, the fraction of Cu atoms and Zn atoms in the
kesterite phase is obtained. This, along with the overall elemental
stoichiometry, allows for determination of the ratio of Cu to Zn in
the kesterite phase. This is clearly distinguished from a mixture
of separate sulfide phases producing the same elemental ratios in
aggregate. The kesterite and stannite family of minerals is
discussed further in sources such as Hall, S. R. et al, Canadian
Minerologist, 16 (1978) 131-137.
[0024] In one embodiment of this invention, there is provided a
nanoparticle that contains copper, zinc, tin and a chalcogen,
wherein the chalcogen is selected from the group consisting of
sulfur, selenium and mixtures thereof. As used herein, the term
"chalcogen" refers to Group 16 elements, and the terms "metal
chalcogenides" or "chalcogenides" refer to semiconductor materials
comprised of metals and Group 16 elements. Herein the term
"binary-metal chalcogenide" refers to a chalcogenide composition
comprising one metal. The term "ternary-metal chalcogenide" refers
to a chalcogenide composition comprising two metals. The term
"quaternary-metal chalcogenide" refers to a chalcogenide
composition comprising three metals. The term "multinary-metal
chalcogenide" refers to a chalcogenide composition comprising two
or more metals and encompasses ternary and quaternary metal
chalcogenide compositions. Metal chalcogenides are useful candidate
materials for photovoltaic applications, since many of these
compounds have optical band gap values well within the terrestrial
solar spectra.
[0025] The particle of copper, zinc, tin and a chalcogen that is
provided herein is a nanoparticle that has a longest dimension of
about 1 nm to about 1000 nm. Herein the terms "nanoparticle",
"nanocrystal" and "nanocrystalline particle" are used
interchangeably to refer to particles having a crystalline
structure and a variety of shapes including spheres, rods, wires,
tubes, flakes, whiskers, rings, disks and triangles, that are
characterized by a longest dimension of about 1 nm to about 1000
nm, preferably, about 5 nm to about 500 nm, and most preferably
about 10 nm to about 100 nm, where the prefix "nano" refers to a
size (as set forth) in the nano range. The longest dimension of a
nanoparticle is defined as the measurement of a nanoparticle from
end to end along the longest dimension, and this dimension will
depend on the shape of the particle. For example, for particles
that are, or are substantially, spheroid, the longest dimension
will be a diameter of a circle as defined by the particle. For
other irregularly-shaped particles (e.g. crystals that may have
angular shapes), the longest dimension may be a diagonal or a side
such that the longest dimension is the furthest distance between
any two points on the surface of the particle. Methods for
determining the longest dimension of a nanoparticle are discussed
further in sources such as "Nanoparticles: From Theory to
Application", G. Schmid, (Wiley-VCH, Weinheim, 2004); and
"Nanoscale Materials in Chemistry", K. J. Klabunde,
(Wiley-Interscience, New York, 2001). In certain particular
embodiments of this invention, the nanoparticles provided herein
are triangular in shape.
[0026] In another embodiment hereof, the nanoparticle of copper,
zinc, tin and chalcogen that is provided can incorporate therein a
capping agent, which can be a group or ligand that is physically
absorbed or adsorbed, or is chemically bonded to the surface of the
particle. The capping agent functions as a type of monolayer or
coating on the particle and may serve as a dispersing aid. Where a
ligand functions as a capping agent, a metal complex is often
formed wherein a metal is bonded to a surrounding array of
molecules or anions. The atom within a ligand that is directly
bonded to the central atom or ion is called the donor atom and
often comprises nitrogen, oxygen, phosphorus or sulfur. The ligand
donates at least one pair of electrons to the metal atom. Examples
of capping agents suitable for use herein include any one or more
of the following: [0027] (a) organic molecules that contain
functional groups such as nitrogen-, oxygen-, sulfur-, or
phosphorus-based functional groups; [0028] (b) a Lewis base; in
various embodiments, the Lewis base can be chosen such that it has
a boiling temperature at ambient pressure that is greater than or
equal to about 150.degree. C., and/or can be selected from the
group consisting of: organic amines, phosphine oxides, phosphines,
thiols, and mixtures thereof; [0029] (c) an electron pair-donor
group, or a group that can be converted into an electron pair-donor
group, and that has a boiling point of less than about 150.degree.
C. at ambient pressure; [0030] (d) primary, secondary or tertiary
amine groups or amide groups, nitrile groups, isonitrile groups,
cyanate groups, isocyanate groups, thiocyanate groups,
isothiocyanate groups, azide groups, thiogroups, thiolate groups,
sulfide groups, sulfinate groups, sulfonate groups, phosphate
groups, phosphine groups, phosphite groups, hydroxyl groups,
alcoholate groups, phenolate groups, ether groups, carbonyl groups
and carboxylate groups; [0031] (e) carboxylic acid, carboxylic acid
anhydride, and glycidyl groups; ammonia, methyl amine, ethyl amine,
butylamine, tetramethylethylene diamine, acetonitrile, ethyl
acetate, butanol, pyridine, ethanethiol, tetrahydrofuran, and
diethyl ether; [0032] (f) the solvent in which the nanoparticle is
formed, such as oleylamine; and/or [0033] (g) a member of the group
consisting of amines, amides, nitriles, isonitriles, cyanates,
isocyanates, thiocyanates, isothiocyanates, azides, thiocarbonyls,
thiolates, sulfides, sulfinates, sulfonates, phosphates,
phosphines, phosphites, hydroxyls, alcoholates, phenolates, ethers,
carbonyls, carboxylates, carboxylic acids, carboxylic acid
anhydrides, glycidyls, and mixtures thereof.
[0034] In yet another embodiment, this invention provides a
composition that contains a plurality of nanoparticles as described
above. Such a composition can, for example, have particle size
distribution such that the average longest dimension of the
particles is in the range of about 10 nm to about 100 nm with a
standard deviation of about 10 nm or less.
[0035] A nanoparticle as provided herein can be made in a reaction
mixture in a solvent such as an organic solvent. Suitable organic
solvents include Lewis bases and organic solvents capable of
forming a Lewis base. The purpose of the solvent is to provide a
medium for the reaction and to assist in minimizing or preventing
agglomeration of the nanoparticles. Suitable organic solvents that
serve as both the solvent and the capping agent include solvents
with Lewis basic functionality. A Lewis basic solvent is useful for
such purpose as it frequently, but not always, succeeds in
providing a coordinating media that covers the surface of
nanoparticles and keeps them from agglomerating. Some specific
classes of suitable organic solvents include organic amines,
phosphine oxides, phosphines and thiols. Preferred solvents are
alkyl amines, with the group consisting of dodecylamine, tetradecyl
amine, hexadecyl amine, octadecyl amine, oleylamine, and
trioctylamine being more preferred, and oleylamine being
particularly preferred. Solvents suitable for use herein include
those having a boiling temperature at ambient pressure greater than
or equal to about 150.degree. C. For example, Lewis basic solvents
with a boiling temperature at ambient pressure greater than or
equal to about 150.degree. C. are also useful, with the group
consisting of organic amines, phosphine oxides, phosphines and
thiols with a boiling point temperature at ambient pressure greater
than or equal to about 150.degree. C. being more preferred, and
alkyl amines with a boiling temperature at ambient pressure greater
than or equal to about 150.degree. C. being particularly
preferred.
[0036] Metal salts and/or complexes of copper, zinc and tin are
used as the source of copper, zinc and tin. These may include
copper complexes, zinc complexes, and tin complexes of one or more
organic ligand(s). Suitable metal salts and/or complexes also
comprise salts and complexes of copper(I), copper(II), zinc(II),
tin(II) and tin(IV) with organic and/or inorganic counterions and
ligands. Metal salts and/or complexes comprising copper(I),
copper(II), zinc(II), tin(II) and tin(IV) halides, acetates, and
2,4-pentanedionates are particularly preferred. Suitable chalcogen
sources or precursors are elemental sulfur, elemental selenium or a
mixture. An advantage of using elemental chalcogen as the chalcogen
source is that it facilitates using the reaction mixture itself as
an ink or ink precursor without further purification. Copper(I),
copper(II), zinc(II), tin(II) and tin(IV) complexes with organic
ligands are preferred for applications in which the reaction
mixture will be used as the ink or ink precursor. Copper(I),
copper(II), zinc(II), tin(II) and tin(IV) complexes of acetates and
2,4-pentanedionates are particularly preferred for this
application.
[0037] Other suitable copper salts and/or complexes include those
selected from the group consisting of: copper(I) halides, copper(I)
acetates, copper(I) 2,4-pentanedionates, copper(II) halides,
copper(II) acetates, and copper(II) 2,4-pentanedionates; the zinc
salts and or complexes are selected from the group consisting of:
zinc(II) halides, zinc(II) acetates, and zinc(II)
2,4-pentanedionates; and the tin salts and/or complexes are
selected from the group consisting of: tin(II) halides, tin(II)
acetates, tin(II) 2,4-pentanedionates, tin(IV) halides, tin(IV)
acetates, and tin(IV) 2,4-pentanedionates.
[0038] As mentioned above, the term "metal complexes" as used
herein refers to compositions wherein a metal is bonded to a
surrounding array of molecules or anions, typically called
"ligands" or "complexing agents". The atom within a ligand that is
directly bonded to the central atom or ion is called the donor atom
and often comprises nitrogen, oxygen, phosphorus, or sulfur. A
ligand donates at least one pair of electrons to the metal atom.
Herein the term "complexes of one or more organic ligand(s)" refers
to metal complexes comprising at least one organic ligand,
including metal acetates and metal acetylacetonates, also referred
to as "2,4-pentanedionates". The term "metal salts" refers to
compositions wherein metal cations and inorganic anions are joined
by ionic bonding. Relevant classes of inorganic anions comprise
oxides, sulfides, carbonates, sulfates and halides.
[0039] A heating mantle, heating block, or oil/sand bath can be
used to heat the flask containing the reaction mixture.
Alternatively, the heating may be carried out with microwave
radiation. A magnetic stirrer is usually placed inside the flask to
keep the reaction mixture well mixed. Optionally, following
synthesis, nanoparticles may be separated from non-ligated solvent
and reaction by-products via precipitation with one or more
nonsolvent or antisolvents. Preferred antisolvents for
precipitation of nanoparticles are organic protic solvents or
mixtures thereof, with methanol and ethanol being particularly
preferred.
[0040] In another embodiment of the processes hereof, individual
metal salts of copper, zinc and tin are dissolved separately in a
solvent such as oleylamine to form solutions. The solutions are
mixed to form a mixture, which is kept at about 100.degree. C. To
the mixture, a solution of elemental chalcogen, such as sulfur, in
solvent is added. The temperature is raised to 230.degree. C. where
the reaction color changes from pale brown to black, indicating the
formation of nanoparticles. The reaction heating is turned off
after 10 minutes, and the system is allowed to cool down for 1
hour. Then the reaction is quenched with an organic polar protic
anti-solvent such as ethanol or mixtures of solvents and
anti-solvents, and the solution is centrifuged. The solid migrates
to the bottom of the tube and is collected by decanting the
supernatant. The solid material is re-suspended at desired
concentrations in solvents or mixtures of solvents.
[0041] In another embodiment of the processes hereof, the reactant
components are added consecutively in sequence. That is, individual
salts or complexes of copper, zinc and tin are dissolved separately
in a solvent to form solutions. Elemental chalcogen is dissolved in
a solvent to form a chalcogen solution. The tin solution and the
zinc solution are mixed to form a binary solution. The copper
solution is mixed with the binary solution to form a tertiary
solution. In a subsequent step the tertiary solution is heated to
between 160 and 230.degree. C. under argon. The chalcogen solution
is mixed with the tertiary solution at a selected temperature from
above to form a quaternary solution. Subsequently, the quaternary
solution is cooled to form copper zinc tin sulfide nanoparticles.
The copper zinc tin sulfide nanoparticles are separated from the
solvent by adding a nonsolvent such as an organic solvent. The
organic solvent may be a polar, protic organic solvent or a binary
mixture of a polar, protic organic solvent and a non-polar or polar
aprotic organic solvent. The copper zinc tin sulfide nanoparticles
are separated from the organic solvent by centrifugation and
decanting the supernatant.
[0042] In another embodiment of the processes hereof, individual
metal salts and/or complexes of copper, zinc and tin can be
separately added in sequence to a mixture of a solvent and a
capping agent to form a reaction mixture, followed by the addition
to the reaction mixture of a chalogen precursor. Where a solvent
and capping agent are present together in a mixture, the capping
agent will typically be selected to a Lewis base, and the solvent
will typically be selected to either not be a Lewis base, or to be
a Lewis base that binds less strongly to the nanoparticle than the
capping agent.
[0043] In this invention, it is advantageous to obtain nanoparticle
formation at lower than typical temperatures, with temperatures of
about 130 to about 300.degree. C. being preferred, about 160 to
about 230.degree. C. being more preferred, and about 160 to about
200.degree. C. being most preferred. At about 160 to about
200.degree. C., the processes described herein frequently gives
nanoparticles having a longest dimension in the size range of about
10 nm to about 100 nm. These low temperatures provide the
advantages of carrying out the procedures at atmospheric or near
atmospheric pressure, and of using lower-boiling solvents and
capping agents. This will result in fewer carbon-based impurities
in annealed films. The more preferred temperatures of about 160 to
about 230.degree. C. are lower than temperatures typically reported
for production of metal chalcogenide nanoparticles, thus also
resulting in energy savings.
[0044] Preventing oxygen and/or water from being present in the
reaction medium during the synthesis of the chalcogenide
nanoparticles, particularly due to the possible formation of metal
oxides, is desirable. Special techniques and equipment are
available to achieve an oxygen-free atmosphere. As such, the
reactant(s) can be prepared in solution(s) in an oxygen-free
atmosphere or inside a glove box, for instance, by using a Schlenk
line or vacuum line connected to a condenser and round bottom
flask. If the introduction of oxygen into the system is
unavoidable, however, for example during the addition of solvents
or precursor solution to the reaction flask, it may be necessary to
degas and/or purge the system with inert gas (e.g. N.sub.2, Ar, or
He) to remove the oxygen before proceeding to further steps.
Although it may be useful to conduct the reactions herein under
oxygen-free atmospheric conditions, such an oxygen-free environment
is not required herein and should not be viewed as limiting the
scope of the present teachings, as we report herein the formation
of CZTS nanoparticles under conditions that limited, but did not
necessarily exclude, the presence of oxygen and water.
[0045] The processes described herein are characterized by a
desirable simplicity of the synthesis process. More particularly,
the disclosed reaction is very fast, such that the crystalline CZTS
nanoparticles are formed within a few minutes after the
constituting precursors are added. In addition, the synthesis of
the nanoparticles is performed at a moderate temperature near
atmospheric pressure. The synthesis is tolerant of the presence of
oxygen. That is, in a number of cases, nanoparticles were formed
under an atmosphere of argon, but the precursor solutions were
prepared in air and not degassed. Furthermore, the precursors used
for the synthesis of the chalcogenide nanoparticles are commonly
available, of low toxicity, and are easy to handle. Lastly, the
equipment needed for the synthesis methods is commonly available
as, for example, special equipment is not needed to achieve high
temperatures and pressures.
[0046] In yet another embodiment of this invention, the
nanoparticles produced by the processes described herein are highly
monodispersed.
[0047] The processes described herein can be used to synthesize
quaternary metal chalcogenide CZTS nanoparticles. The CZTS
nanoparticles prepared herein include crystalline particles having
narrow size distributions that form stable dispersions within
non-polar solvents. Particles as made by the processes hereof were
characterized via XRD, TEM, ICP-MS, EDAX, DLS, and AFM. DLS
revealed a very narrow size distribution within each category (such
as 10.+-.10 nm and 50.+-.10 nm). XRD confirmed the presence of the
kesterite structure. ICP and EDAX indicated that the consecutive
addition of reactants and the use of chalcogen as a precursor gives
more accurate CZTS/Se stoichiometry versus reaction conditions
utilizing simultaneous addition or other sources of chalcogen such
as thiourea.
[0048] The processes described herein can be used to prepare a
composition comprising a quaternary nanoparticle characterized by a
kesterite structure and an average longest dimension of about 1 nm
to about 1000 nm, or a composition comprising a quaternary
nanoparticle that is comprised of copper, tin, zinc and one or more
chalcogen(s) and that is characterized by an average longest
dimension of about 1 nm to about 1000 nm.
[0049] The processes described herein can be used to prepare a
nanoparticle that contains copper, zinc, tin, a chalcogen and a
capping agent, wherein the chalcogen is selected from the group
consisting of sulfur, selenium and mixtures thereof, and the
nanoparticle has a longest dimension of about 1 nm to about 1000
nm. Other embodiments of the processes of this invention are
characterized by the ability to prepare nanoparticles having one or
more of the following features:
[0050] a kesterite structure;
[0051] a molar ratio of copper to zinc to tin to chalcogen that is
about 2:1:1:4;
[0052] a molar ratio of copper to zinc plus tin that is less than
one;
[0053] a molar ratio of zinc to tin is greater than one;
[0054] the longest dimension is about 10 nm to about 100 nm;
[0055] a dopant, such as sodium, is present;
[0056] a capping agent comprises a Lewis base;
[0057] a capping agent comprises a Lewis base wherein the boiling
temperature of the Lewis base at ambient pressure is greater than
or equal to about 150.degree. C., and the Lewis base is selected
from the group consisting of: organic amines, phosphine oxides,
phosphines, thiols, and mixtures thereof;
[0058] a capping agent comprises oleylamine;
[0059] a capping agent has a boiling point of less than about
150.degree. C. at ambient pressure and comprises at least one
electron pair-donor group or group which can be converted into an
electron pair-donor group; and/or
[0060] a capping agent is selected from the group consisting of:
amines, amides, nitriles, isonitriles, cyanates, isocyanates,
thiocyanates, isothiocyanates, azides, thiocarbonyls, thiolates,
sulfides, sulfinates, sulfonates, phosphates, phosphines,
phosphites, hydroxyls, alcoholates, phenolates, ethers, carbonyls,
carboxylates, carboxylic acids, carboxylic acid anhydrides,
glycidyls, and mixtures thereof.
[0061] The processes described herein can also be used to prepare a
composition that contains a plurality of nanoparticles as described
above, and such a composition of nanoparticles can have an average
longest dimension of about 10 nm to about 100 nm with a standard
deviation of about 10 nm or less.
[0062] In another embodiment, this invention further provides inks
that contain CZTS nanoparticles (as described above and/or as
prepared by the processes described above) and one or more organic
solvent(s). In various embodiments, the reaction mixture of the
starting metal components and solvent can itself be used as an ink
without further purification. Optionally, following precipitation
from a reaction mixture, nanoparticles may be re-suspended at the
desired concentration in a solvent or mixture of solvents to give
an ink Preferred organic solvents comprise aromatics, alkanes,
nitriles, ethers, ketones, esters and organic halides or mixtures
thereof with toluene, p-xylene, hexane, heptane, chloroform,
methylene chloride and acetonitrile being particularly preferred.
Preferred concentrations of nanoparticles in the solvents are about
1 wt % to about 70 wt % with about 5 wt % to about 50 wt % being
more preferred, and about 10 wt % to about 40 wt % being most
preferred.
[0063] In addition to CZTS nanoparticles and an organic solvent,
the ink may optionally further comprise one or more chemicals
including without limitation dispersants, surfactants, polymers,
binders, cross-linking agents, emulsifiers, anti-foaming agents,
dryers, fillers, extenders, thickening agents, film conditioners,
anti-oxidants, flow agents, leveling agents, and corrosion
inhibitors, and mixtures thereof. Preferred additives are
oligomeric or polymeric binder(s) and mixtures thereof and/or a
surfactant. Preferably, oligomeric or polymeric binders are present
in an amount of about 20 wt % or less, more preferably about 10 wt
% or less, even more preferably about 5 wt % or less with about 2
wt % or less being most preferred. These oligomeric or polymeric
binders may have a variety of architectures including linear,
branched, comb/brush, star, hyperbranched and/or dendritic
architectures.
[0064] A preferred class of oligomeric or polymeric binders
comprises decomposable binders with decomposition temperatures of
preferably about 250.degree. C. or less, more preferably about
200.degree. C. or less. Preferred classes of decomposable
oligomeric or polymeric binders comprise homo- and co-polymers of
polyethers, polylactides, polycarbonates, poly[3-hydroxybutyric
acid], and polymethacrylate. More preferred decomposable oligomeric
or polymeric binders comprise poly(methacrylic) copolymers,
poly(methacrylic acid), poly(ethylene glycol), poly(lactic acid),
poly[3-hydroxybutryic acid], poly(DL-lactide/glycolide),
poly(propylene carbonate) and poly(ethylene carbonate). Especially
preferred decomposable binders comprise the group consisting of
Elvacite.RTM. 2028 binder and Elvacite.RTM. 2008 binder (Lucite
International, Inc.). Preferably, decomposable oligomeric or
polymeric binders are present in an amount of about 50 wt % or
less, more preferably about 20 wt % or less, even more preferably
about 10 wt % or less with about 5 wt % or less being most
preferred.
[0065] If present in the nanoparticle ink, a surfactant is
preferably present in amounts of about 10 wt % or less, more
preferably about 5 wt % or less, and even more preferably about 3
wt % or less, with 1 wt % or less being most preferred. Numerous
surfactants are available that are suitable for this purpose.
Selection can be based upon the observed coating and dispersion
quality and/or desired adhesion to the substrate. In certain
embodiments, the surfactants comprise siloxy-, fluoryl-, alkyl-and
alkynyl-substituted surfactants. These include, for example,
Byk.RTM. surfactants (Byk Chemie), Zonyl.RTM. surfactants (DuPont),
Triton.RTM. surfactants (Dow), Surfynol.RTM. surfactants (Air
Products) and Dynol.RTM. surfactants (Air Products). A preferred
class of surfactants comprises cleavable or decomposable
surfactants and surfactants with a boiling point below about
250.degree. C., preferably below about 200.degree. C., and more
preferably below about 150.degree. C. A preferred low boiling
surfactant is Surfynol.RTM. 61 surfactant from Air Products.
Cleavable surfactants are further discussed in sources such as
"Cleavable Surfactants", Hellberg et al, Journal of Surfactants and
Detergents, 3 (2000) 81-91; and "Cleavable Surfactants",
Tehrani-Bagha et al, Current Opinion in Colloid and Interface
Science, 12 (2007) 81-91.
[0066] The ink may optionally further comprise other semiconductors
and dopants. Preferred dopants are binary semiconductors, elemental
chalcogens, and sodium. If present in the ink, dopants are
preferably present at about 10 wt % or less, more preferably about
5 wt % or less, and most preferably at about 2 wt % or less.
[0067] In one particular embodiment, this invention thus provides
an ink that contains nanoparticles characterized by a kesterite
structure and/or an average longest dimension of about 1 nm to
about 1000 nm. In another embodiment, this invention provides an
ink comprising nanoparticles that are comprised of copper, tin,
zinc, one or more chalcogen(s) and a capping agent. The
nanoparticles comprised of copper, tin, zinc, one or more
chalcogen(s) and a capping agent may also be characterized by a
kesterite structure and/or an average longest dimension of about 1
nm to about 1000 nm.
[0068] In other embodiments, this invention also provides an ink
that contains one or more organic solvents together with a
composition of a plurality of nanoparticles wherein a nanoparticle
in the composition contains copper, zinc, tin, a chalcogen, and a
capping agent, wherein the chalcogen is selected from the group
consisting of: sulfur, selenium. In yet other embodiments, the
composition of nanoparticles in an ink, or a nanoparticle in the
composition, is characterized by one or more of the following
features: [0069] a nanoparticle in the composition has a kesterite
structure; [0070] a nanoparticle in the composition has a molar
ratio of copper to zinc to tin to chalcogen of about 2:1:1:4;
[0071] a nanoparticle in the composition has a molar ratio of
copper to zinc plus tin of less than one; [0072] a nanoparticle in
the composition has a molar ratio of zinc to tin of greater than
one; [0073] a nanoparticle has a longest dimension of about 1 nm to
about 1000 nm; [0074] a nanoparticle in the composition contains a
capping agent having an electron pair-donor group, or group which
can be converted into an electron pair-donor group, selected from
the group consisting of: amines, phosphine oxides, phosphines,
thiols, amides, nitriles, isonitriles, cyanates, isocyanates,
thiocyanates, isothiocyanates, azides, thiocarbonyls, thiolates,
sulfides, sulfinates, sulfonates, phosphates, phosphites,
hydroxyls, alcoholates, phenolates, ethers, carbonyls,
carboxylates, and mixtures thereof; [0075] a nanoparticle in the
composition contains a capping agent that is oleylamine; [0076] the
composition has a particle size distribution characterized by an
average longest particle dimension of about 10 nm to about 100 nm;
[0077] the concentration of nanoparticles in the ink is between
about 1 wt % to about 70 wt %, as based upon the total weight of
the ink; [0078] the organic solvent in the ink is selected from the
group consisting of aromatics, alkanes, nitriles, ethers, ketones,
esters, organic halides and mixtures thereof; [0079] the ink also
contains constituents selected from the group consisting of
dispersants, surfactants, polymers, binders, crosslinking agents,
emulsifiers, anti-foaming agents, driers, fillers, extenders,
thickening agents, film conditioners, anti-oxidants, flow agents,
leveling agents, corrosion inhibitors and mixtures thereof; [0080]
the ink contains one or more binders or surfactants selected from
the group consisting of decomposable binders; decomposable
surfactants; cleavable surfactants; surfactants with a boiling
point less than about 250.degree. C.; and mixtures thereof; [0081]
the ink contains one or more decomposable binders selected from the
group consisting of homo- and co-polymers of polyethers; homo- and
co-polymers of polylactides; homo- and co-polymers of
polycarbonates; homo- and co-polymers of poly[3-hydroxybutyric
acid]; homo- and co-polymers of polymethacrylates; and mixtures
thereof; and/or [0082] the ink also contains a dopant selected from
the group consisting of binary semiconductors, elemental
chalcogens, sodium, and mixtures thereof.
[0083] A composition or ink of nanoparticles, as described above,
can be formed into a film on a substrate, and such a film may have
one or more layers where some or all of the layers are formed from
an ink or composition hereof. The substrate, which forms a base on
which the layer(s) of film are deposited or disposed, may be
flexible or rigid. The substrate may, for example, be prepared from
aluminum foil or a polymer, which is then used as a flexible
substrate in a roll-to-roll manner (either continuous or segmented)
using a commercially available web coating system. A rigid
substrate may be comprised of at least one material selected from
the group consisting of glass, solar glass, low-iron glass, green
glass, soda-lime glass, steel, stainless steel, aluminum, polymer,
ceramic, metal plates, metallized ceramic plates, metallized
polymer plates, metallized glass plates, and/or any single or
multiple combination of the aforementioned.
[0084] A film of a composition or ink hereof can be formed on a
substrate by any of a variety of solution-based coating techniques
including but not limited to wet coating, spray coating, spin
coating, doctor blade coating, contact printing, top feed reverse
printing, bottom feed reverse printing, nozzle feed reverse
printing, gravure printing, microgravure printing, reverse
microgravure printing, comma direct printing, roller coating, slot
die coating, meyerbar coating, lip direct coating, dual lip direct
coating, capillary coating, ink-jet printing, jet deposition, spray
deposition, and the like. Other coating techniques useful for film
formation include flexographic printing, offset printing, screen
printing, and heat transfer printing.
[0085] The nanoparticles provided by this invention have
semiconductor properties, and unlike conventional semiconductor
materials, the nanoparticles hereof tend to interact and
agglomerate in a colloidal suspension. The incorporation of a
capping agent into the nanoparticles is useful to help stabilize
the colloidal suspension and prevent its decomposition. The
presence of the capping agent will help prevent interaction and
agglomeration of the nanoparticles, thereby maintaining a uniform
distribution of the colloidal substance (e.g. metal chalcogenide
nanoparticles), the disperse phase, throughout the dispersion
medium. Unfortunately, when a nonvolatile capping agent is used,
and when a composition or ink of the nanoparticles is formed into a
film, the nonvolatile capping agent tends to decompose rather than
volatilize, thereby introducing substantial impurities (e.g.
carbon) into the film. Although such impurities are not necessarily
fatal to film performance, they degrade its electronic properties.
In contrast to nonvolatile capping agents, volatile capping agents
are typically driven off during deposition of an ink to form a
film, instead of breaking down and introducing impurities into the
film.
[0086] An advantage of this invention is that use of the moderate
reaction temperatures, as described above, enables the
incorporation of relatively low-boiling capping agents that are not
so volatile that they are not retained during the reaction to form
the nanoparticles, but that are volatile enough to driven off
during film formation. Whichever capping agent is selected for use
during nanoparticle formation, however, it may be exchanged for a
capping agent having greater volatility in a further process step.
For example, a high boiling, nonvolatile capping agent as
incorporated during nanoparticle synthesis may be exchanged for a
volatile capping agent, such as a volatile coordinating Lewis base,
following the synthesis of the nanoparticles. Thus, in one
embodiment, a wet nanoparticle pellet stabilized by a nonvolatile
capping agent as incorporated during synthesis is suspended in a
volatile capping agent to produce a colloidal suspension in which
exchange of capping agents has occurred. When the colloidal
suspension is then deposited on a substrate, it forms a
substantially carbon-free precursor film as the volatile capping
agent that had displaced the nonvolatile capping agent is liberated
from the suspension during film formation.
[0087] In yet another embodiment, an exchange of capping agents can
be effected following film formation. An unannealed film can be
soaked in a bath in a volatile capping agent, which then exchanges
with the nonvolatile capping agent that is incorporated into the
as-synthesized nanoparticles. The nonvolatile capping agent is then
removed along with the excess volatile capping agent when the film
is removed from the bath. Advantages of this method include film
densification along with lower levels of carbon-based impurities in
the film, particularly if and when it is later annealed. Another
advantage of this method is that it is relatively insensitive to
the presence of water, which may cause destabilization of the
colloidal suspension, agglomeration, and colloid decomposition.
[0088] In yet another embodiment, a reaction mixture of
nanoparticles that contain a first capping agent can be contacted
with a second capping agent that has greater volatility than the
first capping agent to exchange in the nanoparticle the second
capping agent for the first capping agent; or a nanoparticle that
contains a first capping agent can be recovered from a reaction
mixture followed by contacting the nanoparticle with a second
capping agent that has greater volatility than the first capping
agent to exchange in the nanoparticle the second capping agent for
the first capping agent. In yet another embodiment, a film that
contains a first capping agent can be contacted with a second
capping agent that has greater volatility than the first capping
agent to exchange in the nanoparticles of the film the second
capping agent for the first capping agent. In either of the above
embodiments, the second capping agent can have a boiling point of
less than about 200.degree. C. at ambient pressure.
[0089] A capping agent is considered volatile in the context of
this invention if it is sufficiently volatile that, instead of
decomposing and introducing impurities when a compositin or ink of
nanoparticles is formed into a film, it evolves during film
deposition. Capping agents having volatility suitable for use
herein include those having a boiling point less than about
200.degree. C. at ambient pressure, preferably less than about
150.degree. C. at ambient pressure, more preferably less than about
120.degree. C. at ambient pressure, and most preferably less than
about 100.degree. C. at ambient pressure, where each of those
ranges is bounded on the lower end by a nonzero value. Other
volatile capping agents suitable for use herein include compounds
that contain at least one electron pair-donor group or a group
which can be converted into such an electron pair-donor group. The
electron pair-donor group can be electrically neutral or negative,
and usually contains atoms such and 0, N, P or S. Electron
pair-donor groups include without limitation primary, secondary or
tertiary amine groups or amide groups, nitrile groups, isonitrile
groups, cyanate groups, isocyanate groups, thiocyanate groups,
isothiocyanate groups, azide groups, thiogroups, thiolate groups,
sulfide groups, sulfinate groups, sulfonate groups, phosphate
groups, phosphine groups, phosphite groups, hydroxyl groups,
alcoholate groups, phenolate groups, ether groups, carbonyl groups
and carboxylate groups. Groups that can be converted into an
electron pair donor groups include, for example, carboxylic acid,
carboxylic acid anhydride, and glycidyl groups. Specific examples
of suitable volatile capping agents include without limitation
ammonia, methyl amine, ethyl amine, butylamine, tetramethylethylene
diamine, acetonitrile, ethyl acetate, butanol, pyridine,
ethanethiol, tetrahydrofuran, and diethyl ether. Preferably, the
volatile capping agent is acetonitrile, butylamine,
tetramethylethylene diamine, or pyridine.
[0090] Although solvent and possibly dispersant are typically
removed from a composition or ink hereof by the drying that occurs
during film formation, a film as formed herein may be annealed by
heating. Heating a film as formed herein typically provides a solid
layer at temperatures much lower than a corresponding layer of
microparticles, which may be caused in part by the greater surface
area contact between particles. In any event, suitable temperatures
at which to anneal a film as formed herein include those between
about 100.degree. C. to about 1000.degree. C., more preferably
between about 200.degree. C. and about 600.degree. C., and even
more preferably between about 375.degree. C. and about 525.degree.
C., this latter range being a safe temperature range for processing
on aluminum foil or high-melting-temperature polymer substrates. A
film to be annealed may be heated and/or accelerated via thermal
processing techniques using at least one of the following
processes: pulsed thermal processing, exposure to laser beams, or
heating via IR lamps, and/or similar or related processes. Other
devices suitable for rapid thermal processing also include pulsed
lasers used in adiabatic mode for annealing, continuous wave lasers
(10-30 W typically), pulsed electron beam devices, scanning
electron beam systems and other beam systems, graphite plate
heaters, lamp systems, and scanned hydrogen flame systems. A
non-directed, low density system may also be used. Alternatively,
other heating processes suitable for use herein include the pulsed
heating processes described in U.S. Pat. Nos. 4,350,537 and
4,356,384; and the pulsed electron beam processing and rapid
thermal processing as described in U.S. Pat. Nos. 3,950,187,
4,082,958, and 4,729,962 (each of the above mentioned patents being
by this reference incorporated as a part hereof for all purposes).
The methods of heating described above may be applied singly, or in
single or multiple combinations with each other, and with the above
or other similar processing techniques.
[0091] The annealing temperature can be modulated to oscillate
within a temperature range without being maintained at a particular
plateau temperature. This technique (referred to herein as rapid
thermal annealing, or RTA) is particularly suitable for forming
photovoltaic active layers (sometimes called "absorber" layers) on
metal foil substrates, such as but not limited to aluminum foil.
Details of this technique are described in U.S. patent application
Ser. No. 10/943,685, which is by this reference incorporated herein
for all purposes.
[0092] The annealed film may have increased density and/or reduced
thickness, versus that of the wet precursor layer, since the
carrier liquid and other materials have been removed during
processing. In one embodiment, the film may have a thickness in the
range of about 0.5 microns to about 2.5 microns. In other
embodiments, the thickness of the film may be between about 1.5
microns and about 2.25 microns. The processing of the deposited
layer of composition or ink will fuse the nanoparticles together
and in most instances, remove void space and thus reduce the
thickness of the resulting dense film.
[0093] Sodium can be incorporated into a composition or ink of CZTS
nanoparticles, as provided herein, to improve the qualities of a
film formed therefrom. In a first method, in a multilayer film as
formed on a substrate, one or more layers of a sodium containing
material may be formed above and/or below a layer as formed from
the nanoparticles hereof. The formation of the sodium-containing
layer may occur by solution coating and/or other techniques
including without limitation sputtering, evaporation, CBD,
electroplating, sol-gel based coatings, spray coating, CVD, PVD,
ALD, and the like. Optionally, in a second method, sodium may also
be introduced into the stack of layers in the film by sodium doping
the CZTS nanoparticles in the film formed therefrom. Optionally, in
a third method, sodium may be incorporated into the ink of
nanoparticles itself. For example, the ink may include a sodium
compound with an organic or inorganic counter-ion (such as sodium
sulfide), where the sodium compound that is added into the ink (as
a separate compound) might be present as particles or dissolved.
The sodium thus may be present in either or both of the "aggregate"
form of the sodium compounds (e.g. dispersed particles), and the
"molecularly dissolved" form.
[0094] The three abovementioned methods are not mutually exclusive,
and each may be applied singly or in any single or multiple
combinations with the other(s) to provide a desired amount of
sodium to a stack containing a CZTS film. Additionally, sodium
and/or a sodium-containing compound may be added to the substrate
(e.g. into a molybdenum target). The source of the sodium is not
limited, and can include one or more of the following: any
deprotonated alcohol where the proton is replaced by sodium, any
deprotonated organic or inorganic acid where the proton is replaced
by sodium, sodium hydroxide, sodium acetate, and the sodium salts
of the following acids: butanoic acid, hexanoic acid, octoanoic
acid, decanoic acid, dedecanoic acid, tetradecanoic acid,
hexadecanoic acid, and the like. Other sources include sodium
halides such as sodium fluoride; and other alkali metals such as K,
Rb or Cs may also be used as dopants with similar effect.
[0095] Additionally, the sodium material may be combined with other
elements that can provide a bandgap widening effect such as Al, Ge
and Si. The use of one or more of these elements, in addition to
sodium, may further improve the quality of the absorber layer. The
use of a sodium compound such as Na.sub.2S, or the like, provides
both Na and S to the film, and could be driven in with an anneal
such as provided by an RTA step to give a layer with a bandgap
different from the bandgap of an unmodified CZTS layer or film.
[0096] In yet another embodiment of this invention, a CZTS film can
be formed on a substrate that includes a source of extra chalcogen,
e.g. in the form of a powder containing chalcogen particles or
binary chalcogenide particles. The extra source of chalcogen may be
provided as a discrete layer containing extra source of chalcogen,
or the extra source of chalcogen can be incorporated in the CZTS
composition or ink from which a single layer is printed onto the
substrate. The chalcogen particles may be micron- or
submicron-sized non-oxygen chalcogen (e.g. Se, S) particles, and
may have a longest dimension that is a few hundred nanometers or
less to a few microns in size. The chalcogenide particles may be
micron- or submicron-sized, and include group IB-binary
chalcogenide nanoparticles such as (CuS,Se) and/or group IIA
non-oxide chalcogenide nanoparticles such as Zn(Se, S) and/or group
IVA binary chalcogenide nanoparticles such as Sn(Se, S).sub.2.
Other suitable chalcogen particles or binary chalogenide particles
include those selected from the group consisting of: non-oxygen
chalcogen particles, group IB-binary chalcogenide nanoparticles,
group IIA non-oxide chalcogenide nanoparticles, Se particles, S
particles, CuS particles, CuSe particles, ZnSe particles, ZnS
particles, SnSe.sub.2 particles, SnS.sub.2 particles, and mixtures
thereof.
[0097] To add the source of extra chalcogen, a mixture of the
quaternary chalcogenide nanoparticles hereof, and the extra
chalcogen particles, is placed on a substrate and heated to a
temperature sufficient to melt the extra chalcogen particles to
form a liquid chalcogen. The liquid chalcogen and the quaternary
nanoparticles are heated to a temperature sufficient to react the
liquid chalcogen with the nanoparticles to correct any chalcogen
deficiency in the resulting film and to densify the layer. The film
is then cooled down.
[0098] In some embodiments, a layer of chalcogen particles or
binary chalcogenide particles may be formed below the CZTS film.
This position of the layer still allows the chalcogen particles to
provide a sufficient surplus of chalcogen or other lacking elements
to the CZTS layer, and thus to fully react and correct the
stochiometry of the quaternary particles hereof. Additionally,
since the chalcogen released from the underlying layer may be
rising through the CZTS film, this position of the layer may be
beneficial to generate greater intermixing between elements. The
thickness of the chalcogen rich layer may be in the range of about
0.4 to 0.5 microns. In still another embodiment, the thickness of
the chalcogen rich layer is about 500 to 50 nm. In still other
embodiments hereof, multiple layers of material may be printed and
reacted with chalcogen to differing extents before deposition of
the next layer, and in this manner a graded compositional content
can be provided across the group of layers making up a multi-layer
film.
[0099] The binary chalcogenide particles may be obtained starting
from a binary chalcogenide feedstock material, e.g. micron size
particles or larger. The binary chalcogenide feedstock may be ball
milled to produce particles of the desired size. Binary alloy
chalcogenide particles may alternatively be formed by
pyrometallurgy or by melting elemental components and spraying the
melt to form droplets that solidify into nanoparticles.
[0100] The chalcogen particles may be larger than the binary
chalcogenide nanoparticles and the quaternary chalcogenide
nanoparticles since chalcogen particles melt before the binary and
quaternary nanoparticles and provide good contact with the
material. Preferably the chalcogen particles are smaller than the
thickness of the chalcogenide film that is to be formed. The
chalcogen particles (e.g. Se or S) may be formed in several
different ways. For example, Se or S particles may be formed
starting with a commercially available fine mesh powder (e.g. 200
mesh/75 micron) and ball milling the powder to a desirable size. Se
or S particles may alternatively be formed using an
evaporation-condensation method. Alternatively, Se or S feedstock
may be melted and sprayed ("atomization") to form droplets that
solidify into nanoparticles.
[0101] Chalcogen particles may also be formed using a
solution-based technique such as the "Top-Down" method as described
by Wang and Xia in Nano Letters, 2004 Vol. 4, No. 10, 2047-2050
("Bottom-Up and Top-Down Approaches to Synthesis of Monodispersed
Spherical Colloids of Low Melting-Point Metals"). This technique
allows processing of elements with melting points below 400.degree.
C. as monodispersed spherical colloids, with diameter controllable
from 100 nm to 600 nm, and in copious quantities. For this
technique, chalcogen (Se or S) powder is directly added to boiling
organic solvent, such as di(ethylene glycol) and melted to produce
big droplets. After the reaction mixture has been vigorously
stirred and thus emulsified for 20 min, uniform spherical colloids
of metal are obtained as the hot mixture is poured into a cold
organic solvent bath (e.g. ethanol) to solidify the chalcogen (Se
or Se) droplets.
[0102] In yet another embodiment hereof, an electronic device can
be fabricated from a film comprises multiple layers; and a first
layer can contain a plurality of nanoparticles as described above,
and a second layer can contain a binary semiconductor, a chalcogen
source, a sodium-containing material, or a mixtures thereof. In
such a device, the first layer can be adjacent to the second
layer.
[0103] A film fabricated on a substrate as described above can be
incorporated into an electronic device to serve, for example, as an
absorber layer in a photovoltaic device, module, or solar panel.
The typical solar cell includes a transparent substrate (such as
soda-lime glass), a back contact layer (e.g.
[0104] molybdenum), an absorber layer (also referred to as the
first semiconductor layer), a buffer layer (e.g. CdS; also referred
to as the second semiconductor layer), and a top electrical
contact. The solar cell may also include an electrical contact or
electrode pad on the top contact layer, and an antireflective (AR)
coating on the front surface of the substrate to enhance the
initial transmission of light into the semiconductor material. FIG.
3 illustrates the above features in the stack shown therein, which
contains the following elements: transparent substrate 1; back
contact layer 2; absorber layer 3 (which is formed from the p-type
CZTS/Se nanoparticles hereof); buffer layer 4; top contact layer 5
[which can be, for example, a transparent conducting oxide ("TCO")
such as zinc oxide doped with aluminum]; and the electrical contact
or electrode pad on the top contact layer 6.
[0105] The substrate may be made, for example, of a metal foil,
such as titanium, aluminum, stainless steel, molybdenum, or a
plastic or polymer, such as polyimides (PI), polyamides,
polyetheretherketone (PEEK), polyethersulfone (PES), polyetherimide
(PEI), polyethylene naphthalate (PEN), polyester (PET), or a
metallized plastic. The base electrode may be made of an
electrically conductive material such as a layer of Al foil, e.g.
about 10 microns to about 100 microns thick. An optional
interfacial layer may facilitate bonding of the electrode to the
substrate. The adhesion can be comprised of a variety of materials,
including without limitation chromium, vanadium, tungsten, and
glass, or compounds such as nitrides, oxides, and/or carbides. The
CZTS absorber layer may be about 0.5 micron to about 5 microns
thick after annealing, and more preferably from about 0.5 microns
to about 2 microns thick after annealing.
[0106] The n-type semiconductor thin film (sometimes referred to as
a junction partner layer) may include, for example, inorganic
materials such as cadmium sulfide (CdS), zinc sulfide (ZnS), zinc
hydroxide, zinc selenide (ZnSe), n-type organic materials, or some
combination of two or more of these or similar materials, or
organic materials such as n-type polymers and/or small molecules.
Layers of these materials may be deposited, for example, by
chemical bath deposition (CBD) and/or chemical surface deposition
(and/or related methods), to a thickness ranging from about 2 nm to
about 1000 nm, more preferably from about 5 nm to about 500 nm, and
most preferably from about 10 nm to about 300 nm. This may also
configured for use in a continuous roll-to-roll and/or segmented
roll-to-roll and/or a batch mode system.
[0107] The transparent electrode may include a transparent
conductive oxide layer such as zinc oxide (ZnO), aluminum doped
zinc oxide (ZnO:Al), iindium tin oxide (ITO), or cadmium stannate,
any of which can be deposited using any of a variety of means
including but not limited to sputtering, evaporation, CBD,
electroplating, CVD, PVD, ALD, and the like.
[0108] Alternatively, the transparent electrode may include a
transparent conductive polymeric layer, e.g. a transparent layer of
doped PEDOT (poly-3,4-ethylenedioxythiophene), which can be
deposited using spin, dip, or spray coating, and the like.
PSS:PEDOT is a doped conducting polymer based on a heterocyclic
thiophene ring bridged by a diether. A water dispersion of PEDOT
doped with poly(styrenesulfonate) (PSS) is available from H. C.
Starck of Newton, Mass. under the trade name of Baytron.RTM. P. The
transparent electrode may further include a layer of metal (e.g.,
Ni, Al or Ag) fingers to reduce the overall sheet resistance.
Alternatively, the transparent conductor layer may comprise a
carbon nanotube-based transparent conductor.
[0109] The operation and effects of certain embodiments of the
inventions hereof may be more fully appreciated from a series of
examples as described below. The embodiments on which these
examples are based are representative only, and the selection of
those embodiments to illustrate the invention does not indicate
that materials, components, reactants, configurations, designs,
conditions, specifications, steps, techniques not described in the
examples are not suitable for use herein, or that subject matter
not described in the examples is excluded from the scope of the
appended claims and equivalents thereof.
EXAMPLES
[0110] The following is a list of abbreviations and trade names
used above and in the examples:
TABLE-US-00001 Abbreviation Description XRD X-Ray Diffraction TEM
Transmission Electron Microscopy ICP-MS Inductively Coupled Plasma
Mass Spectrometry AFM Atomic Force Microscopy DLS Dynamic Light
Scattering CIGS Copper-Indium-Gallium-Sulfo-di-selenide CZTS Copper
Zinc Tin Sulfide (Cu.sub.2ZnSnS.sub.4) CZTSe Copper Zinc Tin
Selenide Cu.sub.2ZnSnSe.sub.4) CZTS/Se All possible combinations of
CZTS and CZTSe EDX Energy-Dispersive X-ray Spectroscopy Deg Degree
Oleylamine Cis-1-Amino-9-octadecene
CH.sub.3(CH.sub.2).sub.7CH.dbd.CH(CH.sub.2).sub.7CH.sub.2NH.sub.2
FW Formula Weight Ex Example Elvacite 2028 Methacrylate Copolymer
with Tg of 45.degree. C. and MW of 59,000; Lucite International,
Inc. Elvacite 2008 Methyl Methacrylate Copolymer with Tg of
105.degree. C. and MW of 37,000; Lucite International, Inc. RTA
Rapid Thermal Anneal TCO Transparent Conducting Oxide
Materials
[0111] Cuprous chloride, Cu(I)Cl 99.99%, zinc chloride, ZnCl.sub.2
99.99%; tin chloride, SnCl.sub.2 99.99%, elemental sulfur,
elemental selenium, thiourea, toluene, p-xylene, acetonitrile and
chloroform were all purchased from Aldrich and used without further
purification. Oleylamine, 70% technical grade, was purchased from
Fluka and filtered through a 0.45 .mu.m filter (Whatman GE).
Elvacite.RTM. 2028 and Elvacite.RTM. 2008 were obtained from Lucite
International, Inc. (Cordova, Tenn.).
General Procedure for the Preparation of Solar Cells
[0112] Mo-Sputtered Substrates. Substrates for solar cells were
prepared by coating a soda lime glass substrate with a 500 nm layer
of patterned molybdenum using a Denton Sputtering System.
Deposition conditions were: 150 watts of DC Power, 20 sccm Ar, and
5 mT pressure.
Cadmium Sulfide Deposition.
[0113] Precursor solutions for the CdS bath were prepared according
to Table 1, and were combined at room temperature in the reaction
vessel which contained prepared substrates, such that substrates to
be coated would be fully submerged. Immediately after mixing of the
precursor solutions, the vessel containing the mixed components was
placed in a water-heated vessel (65.degree. C.) (large
crystallization dish). CdS was deposited on the sample for 17.5
minutes. Samples were rinsed in DI water for an hour, and dried at
200.degree. C. for 15 minutes.
TABLE-US-00002 TABLE 1 Precursor solutions for CdS bath.
Concentration FW (g/mol) (mol/L) Volume (mL) Amount (g) Water 18
183 183 NH4OH NA 28% 32.6 CdSO4 208.46 0.015 25 0.0781725 Thiourea
76.12 1.5 12.5 1.42725 Total volume = 253.1
Transparent Conductor Deposition.
[0114] A transparent conductor was sputtered on top of the CdS with
the following structure: 50 nm of insulating ZnO (150 W RF, 5
mTorr, 20 sccm) and 500 nm of Al-doped ZnO using a 2%
Al.sub.2O.sub.3, 98% ZnO target (75 W RF, 10 mTorr, 20 sccm).
Example 1
Synthesis of Copper Zinc Tin Sulfur Nanoparticles with Consecutive
Addition of Precursors with 230.degree. C. Reaction Temperature
[0115] In the following procedure, all metal salts and elemental
sulfur were dissolved in oleylamine at 100.degree. C.: A solution
of 80 mg (0.586 mmol) of zinc chloride dissolved in 10 mL of
oleylamine and a solution of 102 mg (0.587 mmol) of tin(II)
chloride dissolved in 10 mL of oleylamine were mixed with stirring
in a flask that was previously degassed and heated to 110.degree.
C. under an Ar atmosphere. After 5 minutes of stirring, a solution
of 77 mg (0.777 mmol) of cuprous chloride dissolved in 10 mL of
oleylamine was added to the reaction mixture, and the resulting
solution was stirred for an additional 5 minutes. At this point, a
solution of 163 mg (5.08 mmol) of sulfur dissolved in 10 mL of
oleylamine was added to the system, and the reaction temperature
was raised to 230.degree. C. at a rate of 10.degree. C./min.
[0116] After reaching 230.degree. C., the system was maintained at
this temperature for 10 minutes and then the heating was turned
off. The reaction was allowed to cool down naturally with stirring
with the heating block remaining in place as it cooled. Following
cooling, 80 mL of ethanol was added to the reaction mixture. The
particles were collected via centrifugation and decanting of the
solvent. The presence of kesterite structure was determined by XRD
and the ratio of Cu:Zn:Sn:S was determined via ICP (see Table 2).
The particle size was determined using DLS, TEM (see FIGS. 1-1,
1-2, 1-3, 1-4) and AFM. The particle size ranged between 1-10
nm.
Example 2
Synthesis of Copper Zinc Tin Sulfur Nanoparticles with Consecutive
Addition of Precursors with 160.degree. C. Reaction Temperature
[0117] In the following procedure, all metal salts and elemental
sulfur were dissolved in oleylamine at 100.degree. C.: A solution
of 80 mg (0.586 mmol) of zinc chloride dissolved in 10 mL of
oleylamine and a solution of 102 mg (0.587 mmol) of tin(II)
chloride dissolved in 10 mL of oleylamine were mixed with stirring
in a flask that was previously degassed and heated to 160.degree.
C. under an Ar atmosphere. After 5 min of stirring, a solution of
77 mg (0.777 mmol) of cuprous chloride dissolved in 10 mL of
oleylamine was added and the three components were stirred for an
additional 5 minutes. At this point, a solution of 163 mg (5.08
mmol) of elemental sulfur dissolved in 10 mL of oleylamine was
added to the system while the reaction temperature was maintained
at 160.degree. C. for an additional 10 minutes. The heating was
then turned off and the reaction was allowed to cool down naturally
under stirring with the heating block remaining it place as it
cooled down. Upon cooling, 40 mL of a 1:1 mixture of hexane and
ethanol was added to the reaction mixture. The particles were
collected via centrifugation and decanting of the solvent. The
presence of kesterite structure was determined by XRD and the ratio
of Cu:Zn:Sn:S was determined via EDX (see Table 2). The particle
size was determined using DLS, TEM (FIG. 2) and AFM. The particle
sizes ranged from 10-50 nm.
Example 3
Synthesis of Copper Zinc Tin Selenium Nanoparticles with
Consecutive Addition of Precursors
[0118] In the following procedure, all metal salts and elemental
selenium were dissolved in oleylamine at 100.degree. C.: The
procedure of Example 1 was followed with elemental selenium 401 mg
(5.08 mmol) replacing elemental sulfur. The presence of kesterite
structure was determined by XRD.
Example 4
Synthesis of Copper Zinc Tin Sulfur Nanoparticles with Simultaneous
Addition of Precursors
[0119] In the following procedure, all metal salts and elemental
sulfur were dissolved in oleylamine at 100.degree. C.: A solution
of 80 mg (0.586 mmol) of zinc chloride dissolved in 10 mL of
oleylamine, a solution of 102 mg (0.587 mmol) of tin(II) chloride
dissolved in 10 mL of oleylamine, a solution of 77 mg (0.777 mmol)
of cuprous chloride dissolved in 10 mL of oleylamine, and a
solution of 163 mg (5.08 mmol) of elemental sulfur dissolved in 10
mL of oleylamine were mixed simultaneously, and the reaction
temperature was raised to 230.degree. C. at a rate of 10.degree.
C./min. After reaching 230.degree. C., the system was maintained at
this temperature for 10 minutes and then the heating was turned
off. The reaction was allowed to cool down naturally with stirring
with the heating block remaining in place as it cooled down.
Following cooling, 80 mL of ethanol was added to the mixture. The
particles were collected via centrifugation and decanting of the
solvent. The particle sizes ranged from 10-50 nm and the
composition indicated a ratio of 1:2 of Zn:Sn within CZTS. The
presence of kesterite structure was determined by XRD and the ratio
of Cu:Zn:Sn:S was determined via EDX (see Table 2). The particle
size was determined using DLS, TEM and AFM. While maintaining a
similar XRD pattern, a different composition was obtained.
Comparative Example 5
Nanoparticle Synthesis Using Thiourea as the Sulfur Source
[0120] A mixture of 0.1 g of cuprous chloride (1 mmol of Cu), 0.068
g of zinc chloride (0.5 mmol of Zn), 0.094 g of tin(II) chloride
(0.5 mmol of Sn), and 10 mL of oleylamine is vigorously stirred and
degassed in a Schlenk flask for 30 minutes at 60.degree. C. by
pulling vacuum with the Schlenk line. The mixture is then heated to
130.degree. C. under nitrogen for 10 minutes. During heating, the
solution turns from blue to yellow, indicating the formation of
oleylamine complexes of Cu, Zn and Sn. Meanwhile, a thiourea
solution is prepared by dissolving 0.076 g of thiourea (1.0 mmol)
in 1 mL of oleylamine at 200.degree. C. under nitrogen in a Schlenk
flask. The Zn/Sn/Cu/oleylamine reactant solution is cooled to
100.degree. C. and the thiourea reactant solution is added via
cannula. Immediately after injection, the reaction mixture is
heated to 240.degree. C. at a rate of 15.degree. C./min. After 1
hour, the Schlenk flask containing the nanocrystals is removed from
the heating mantle and allowed to cool to room temperature. Ethanol
(30 mL) is then added to precipitate the nanocrystals, followed by
centrifugation at 7000 rpm for 3 minutes. The supernatant is
decanted off of the nanocrystals and discarded. The nanocrystals
redisperse in a variety of non-polar organic solvents, including
chloroform, hexane, and toluene. Prior to characterization,
dispersions are typically centrifuged again at 7000 rpm for 5 min
to remove inadequately capped nanocrystals. The sample was
characterized via XRD and EDAX (see Table 2). The XRD results
reveal the presence of SnS, ZnS, Cu2S. The small amount of Zn
detected by EDAX reflects that this process, which is used to
synthesize other type of chalcogenide-containing materials, cannot
be extrapolated for CZTS synthesis.
TABLE-US-00003 TABLE 2 Cu, Zn, Sn and S Stoichiometry for Examples
1 and 2 with Consecutive Addition of Precursors and Examples 4
(Simultaneous Addition of Precursors) and Comparative Example 5
(Thiourea Precursor) Example Method Cu Zn Sn S 1 ICP 1.87 1.50 1.14
4.00 2 EDAX 1.96 1.56 1.19 4.18 4 EDAX 2.26 0.63 1.30 4.00 5 EDAX
2.00 0.10 4.02 1.54
Example 6
Preparation of CZTS Ink in Toluene
[0121] An ink was prepared by redispersing 0.086 g of CZTS
nanoparticles produced by the procedure of Example 1. The particles
were dispersed via ultrasonication in 1 mL of toluene (density
0.8669 g/mL) to generate an ink with a nanoparticle concentration
of 10 wt %.
Example 7
Preparation of CZTS Ink in p-Xylene with Elvacite.RTM. 2028
Binder
[0122] An ink was prepared by the following steps: First, 0.172 g
of CZTS nanoparticles prepared by the procedure of Example 1 were
dispersed via ultrasonication in 1 mL of p-xylene (density 0.861
g/mL) to generate a 20 wt % suspension of nanoparticles. Then, the
nanoparticle suspension was mixed with 1 mL of a solution of 2 wt %
of Elvacite 2028 p-xylene to generate an ink with 10% wt
concentration of nanoparticles.
Example 8
Preparation of CZTS Ink in Toluene with Elvacite.RTM. 2028
Binder
[0123] An ink was prepared by the following steps: First, 0.172 g
of CZTS nanoparticles prepared by the procedure of Example 1 were
dispersed via ultrasonication in 1 mL of toluene (density 0.861
g/mL) to generate a 20 wt % suspension of nanoparticles. Then, the
nanoparticle suspension was mixed with 1 mL of a solution of 2 wt %
Elvacite.RTM. 2028 in toluene to generate an ink with 10 wt %
concentration of nanoparticles.
Example 9
Preparation of CZTS Ink in Acetonitrile with Elvacite.RTM. 2008
Binder
[0124] An ink was prepared by the following steps: First, 0.172 g
of CZTS nanoparticles prepared by the procedure of Example 1 were
dispersed via ultrasonication in 1 mL of acetonitrile (density
0.786 g/mL) to generate a 20 wt % suspension of nanoparticles.
Then, the nanoparticle suspension was mixed with 1 mL of a solution
of 2 wt % Elvacite.RTM. 2008 in acetonitrile to generate an ink
with 10 wt % concentration of nanoparticles.
Example 10
CZTS Nanoparticle Synthesis: Exchange of Oleylamine with
Butylamine
[0125] CZTS nanoparticles were prepared according to the procedure
of Example 1, redispersed in toluene, centrifuged and then the
solvent was decanted. The pellet was dried under vacuum. The dry
material was then weighed and 0.3 g of material was then placed in
a round bottom flask equipped with stirring bar under an Ar
atmosphere. Anhydrous butyl amine (5 mL) was added to the flask
using a syringe, and the reaction was allowed to stir for 3 days at
room temperature to give an ink in butylamine. Next, the particles
were precipitated by adding 15 mL of ethanol and collected via
centrifugation and decanting of solvent to give a pellet of
nanoparticles.
Example 11
Preparation of Butylamine-Capped CZTS-Nanoparticle Ink in
Chloroform
[0126] The nanoparticle pellet prepared according to Example 10 is
redispersed in chloroform at a concentration of 150 mg/mL and is
then filtered through a 0.22 ium polytetrafluoroethylene filter to
generate an ink
Example 12
Formation of CZTS Film
[0127] Wet pellets of CZTS (.about.100 mg) prepared according to
Example 1 were dissolved in 0.5 mL of toluene, yielding fluid
suspensions. The suspensions were sonicated 5 to 60 minutes using a
Branson 25-10 Sonicator, in steps of 5 minutes. The highest quality
dispersions were obtained with sonication times between 30 and 45
minutes.
[0128] Using a Speed-line Technologies 3GP benchtop spin-coater,
samples were then spin-coated onto glass substrates and also onto
the molybdenum-coated side of glass substrates patterned with
sputtered molybdenum. Coating speeds ranged from 500-1500 rpms and
coating times ranged from 10 to 30 seconds. The highest quality
coatings were obtained for 1000-rpm, 20-sec spinning conditions.
The resulting films varied in thickness between 50 and 800 nm.
[0129] Films were then annealed in either a furnace or a rapid
thermal annealing processor. Annealing temperatures ranged from 400
to 550.degree. C., and times varied from 10-30 minutes. The highest
quality annealed films were obtained by annealing for 15 minutes at
temperatures between 500 and 550.degree. C. Film thickness and
roughness were obtained with a profilometer and films were further
characterized by optical spectra. For Rappid thermal annealing, a
MILA-5000 Infrared Lamp Heating System by ULVAC-RICO Inc. (Methuen,
Mass.) was utilized for heating and the system was cooled using a
Polyscience (Niles, Ill.) recirculating bath held at 15.degree. C.
Samples were heated under nitrogen purge as follows: 20 for 10 min,
ramp to 400.degree. C. in 1 min; hold at 400.degree. C. for 2 min;
cool back to 20.degree. C. during .about.30 min.
Example 13
Densification of CZTS Films through Soaking with Ethanol
[0130] Pre-annealed films were prepared as described in Example 12.
These films were left on the spin coater after being coated and
were then soaked with ethanol. After a few seconds of soaking, the
spinning program was re-run to aid drying of the film. The
resulting films were denser and harder than the pre-soaked
films.
Example 14
Formation of Bar-Coated CZTS Films
[0131] Wet pellets of CZTS (.about.100 mg) prepared according to
Example 1 were dissolved in .about.0.5 mL of various solvents,
including toluene and chloroform to give inks The resulting CZTS
inks were bar-coated onto glass substrates. Two of the bar-coated
films were annealed in a furnace at 550.degree. C. for 15 minutes.
The films were characterized via XRD and some of films were
scrapped and analyzed via HR-TEM.
Example 15
Solar Cell with Spin-Coated CZTS Nanoparticle-Derived Absorber
Layer
[0132] An annealed film of p-type CZTS absorber on Mo-sputtered
soda lime glass was prepared as described in Example 12. The sample
was then placed in a CdS bath and about 50 nm of n-type CdS was
deposited on top of the CZTS film. The transparent conductor was
then sputtered on top of the CdS. The finished device was tested
under 1 sun illumination and the J-V characteristics are
illustrated in FIG. 4
Example 16
CZTS-Based Solar Cell on Polyimide Substrate with Thermally
Evaporated Sodium Layer
[0133] A solar cell is fabricated with a CZTS absorber layer using
high-temperature polyimide film as the substrate. The procedure of
Example 15 is followed with the modification that in order to
provide some of the sodium necessary for high performance solar
cells, a very thin layer of Na (less than 1 nm) is thermally
evaporated on the Mo. The CZTS ink is then coated onto the
polyimide/Mo/Na substrate.
Example 17
Solar Cell on Polyimide Substrate Fabricated from Sodium-Doped CZTS
Ink
[0134] A solar cell is fabricated with a CZTS absorber layer using
high-temperature polyimide film as the substrate according to the
procedure of Examples 15. Sodium octanoate is incorporated into the
ink at 0.5 wt %.
Example 18
CZTS Ink Doped with Elemental Chalcogen
[0135] An ink of CZTS nanoparticles is formed according to the
procedure of Example 6. Submicron-sized elemental sulfur particles
are added to the ink at 1 wt % with sonication. A solar cell is
fabricated with this ink by following the procedure of Example
15.
Example 19
CZTS Ink Doped with Binary Chalcogenide Particles
[0136] An ink of CZTS nanoparticles is formed according to the
procedure of Example 6. Binary chalcogenide nanoparticles of
SnS.sub.2 and ZnS are added to the ink at 0.2 wt % each with
sonication. Utilizing this ink, a coating and solar cell are
fabricated according to the procedure of Example 15.
Examples 20-25
Coatings and Solar Cells Based on a Variety of CZTS Inks
[0137] Inks of CZTS nanoparticles are formed according to the
procedures listed in Table 3. Utilizing these inks, coatings and
solar cells are fabricated according to the procedure of Example
15.
TABLE-US-00004 TABLE 3 Inks utilized in solar cells in Examples
20-25. Ink Prepared According to the Procedure of the Following
Example Examples: 20 Example 2 21 Example 3 22 Example7 23 Example
8 24 Example 9 25 Example 11
Example 26
Variation in CZTS Composition: Sulfur Gradient
[0138] Ten inks of CZTS nanoparticles doped with elemental sulfur
are formed according to the procedure of Example 18 with the weight
percent sulfur in the inks varying from 1.0 to 0.1 wt % with steps
of 0.1 wt %. A CZTS film with a sulfur gradient is created on
Mo-sputtered soda lime glass by spray-coating the CZTS inks in
consecutive spray passes using the ink compositions for each layer
as shown in Table 4. Utilizing this absorber layer, a solar cell is
fabricated according to the procedure of Example 15.
Example 27
Variation in CZTS Composition: Cu-Poor with Cu Gradient
[0139] Ten inks of CZTS nanoparticles doped with binary
chalcogenide nanoparticles of SnS.sub.2 and ZnS are formed
according to the procedure of Example 19 with the weight percent of
binary chalcogenide nanoparticles in the inks varying from 0 to
0.45 wt % with steps of 0.05 wt %. A copper-poor CZTS film with a
copper gradient is created on Mo-sputtered soda lime glass by
spray-coating the CZTS inks in consecutive spray passes using the
ink compositions for each layer as shown in Table 4. Utilizing this
absorber layer, a solar cell is fabricated according to the
procedure of Example 15.
TABLE-US-00005 TABLE 4 Wt % of dopants in CZTS inks used in layers
of gradient films Wt % Additive in Ink Used for Layers 1-10
(L1-L10) Ex L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 26 1.0 0.9 0.8 0.7 0.6
0.5 0.4 0.3 0.2 0.1 27 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
0.45
Example 28
Spray-Coating of CZTS Nanoparticles
[0140] CZTS nanoparticles (100 mg) were prepared according to the
procedure of Example 1. The nanoparticles were suspended in 5 mL of
chloroform and the suspension then sonicated for 10 minutes prior
to deposition. The ink is sprayed on Mo-sputtered soda lime glass
according to the conditions given in Table 5. Utilizing this
absorber layer, solar cells are fabricated according to the
procedure of Example 15.
Example 29
Insulating ZnO Window
[0141] A solar cell is fabricated according to the procedure of
Example 15 utilizing insulating ZnO as the window in place of
CdS.
Example 30
Solar Cell with CZTS Absorber Layer and Carbon Nanotube-Based
Transparent Conductor
[0142] A solar cell is fabricated according to the procedure of
Example 15 utilizing utilizing a carbon nanotube-based transparent
conductor in place of ZnO:Al.
Example 31
CZTS Nanoparticles Synthesized Via Microwave Radiation
[0143] In the following procedure, all metal salts and elemental
sulfur are dissolved in oleylamine at 100.degree. C.: A solution of
80 mg (0.586 mmol) of zinc chloride dissolved in 5 mL of
oleylamine, a solution of 102 mg (0.587 mmol) of tin(II) chloride
dissolved in 5 mL of oleylamine, a solution of 77 mg (0.777 mmol)
of cuprous chloride dissolved in 5 mL of oleylamine and a solution
of 163 mg (5.08 mmol) of elemental sulfur dissolved in 5 mL of
oleylamine were mixed simultaneously and loaded in a microwave
synthesizer special vial equipped with stir bar. The vial is then
loaded into an Initiator-8 microwave (Biotage, Sweden) system with
a heat set point of 230.degree. C. After reaching 230.degree. C.,
the system was maintained at this temperature for 10 minutes and
then the vial is removed from the system. Following cooling down in
the rack for 30 min, the vial is opened and the reaction mixture is
transferred as equal volumes (20 mL each) into two 50 mL Falcon
test tubes. Next, a volume of 20 mL of ethanol is added to each
tube. The particles are collected via centrifugation and decanting
of the solvent.
Example 32
Sulfurization of CZTS Films
[0144] CZTS nanoparticles were prepared according to the procedure
of Example 1. Wet pellets of CZTS (.about.100 mg) prepared
according to Example 1 were suspended in .about.0.5 mL of various
solvents, including toluene and chloroform to give inks The
resulting CZTS inks were bar-coated onto glass substrates.
Bar-coated films were annealed in a furnace at 500.degree. C. for 1
hour under rich sulfur atmosphere and under continuous flow of
nitrogen atmosphere. Annealings were carried out in a single-zone
Lindberg/Blue (Ashville, N.C.) tube furnace equipped with an
external temperature controller and a two-inch quartz tube. The
coated substrates were placed on quartz plates inside of the tube.
A 3-inch long ceramic boat was loaded with 2.5 g of elemental
sulfur and placed near the nitrogen inlet, outside of the direct
heating zone.
Example 33
Selenization of CZTS Films
[0145] CZTS nanoparticles are prepared according to the procedure
of Example 1. Wet pellets of CZTS (.about.100 mg) prepared
according to Example 1 are suspended in .about.0.5 mL of various
solvents, including toluene and chloroform to give inks The
resulting CZTS inks are bar-coated onto glass substrates.
Bar-coated films are annealed in a furnace at 500.degree. C. for 1
hour under rich selenium atmosphere and under continuous flow of
nitrogen atmosphere. The samples are placed in a 5'' L.times.1.4''
W by 1'' H graphite box with 1/8'' walls that is equipped with a
lid with a lip and a 1 mm hole in the center (custom-made by
Industrial Graphite Sales, Harvard, Ill.). Each graphite box is
equipped with two small ceramic boats (0.984'' L.times.0.591''
W.times.0.197'' H) at each end, containing 0.1 g of selenium. The
graphite box is then place in a two-inch tube, with up to two
graphite boxes per tube. Vacuum is applied to the tube for 10-15
min, followed by a nitrogen purge for 10-15 min. This purging
process is carried out three times. The tube containing the
graphite boxes is then heated for 1 hour at 500.degree. C. in the
single-zone furnace with both heating and cooling carried out under
a nitrogen purge. The exiting gas is sparged through consecutive
bubblers: 1 M NaOH (aq) followed by 1 M Cu(NO3)2 (aq).
[0146] Where a range of numerical values is recited or established
herein, the range includes the endpoints thereof and all the
individual integers and fractions within the range, and also
includes each of the narrower ranges therein formed by all the
various possible combinations of those endpoints and internal
integers and fractions to form subgroups of the larger group of
values within the stated range to the same extent as if each of
those narrower ranges was explicitly recited. Where a range of
numerical values is stated herein as being greater than a stated
value, the range is nevertheless finite and is bounded on its upper
end by a value that is operable within the context of the invention
as described herein. Where a range of numerical values is stated
herein as being less than a stated value, the range is nevertheless
bounded on its lower end by a non-zero value.
[0147] In this specification, unless explicitly stated otherwise or
indicated to the contrary by the context of usage, amounts, sizes,
ranges, formulations, parameters, and other quantities and
characteristics recited herein, particularly when modified by the
term "about", may but need not be exact, and may also be
approximate and/or larger or smaller (as desired) than stated,
reflecting tolerances, conversion factors, rounding off,
measurement error and the like, as well as the inclusion within a
stated value of those values outside it that have, within the
context of this invention, functional and/or operable equivalence
to the stated value.
[0148] In this specification, unless explicitly stated otherwise or
indicated to the contrary by the context of usage, where an
embodiment of the subject matter hereof is stated or described as
comprising, including, containing, having, being composed of or
being constituted by or of certain features or elements, one or
more features or elements in addition to those explicitly stated or
described may be present in the embodiment. An alternative
embodiment of the subject matter hereof, however, may be stated or
described as consisting essentially of certain features or
elements, in which embodiment features or elements that would
materially alter the principle of operation or the distinguishing
characteristics of the embodiment are not present therein. A
further alternative embodiment of the subject matter hereof may be
stated or described as consisting of certain features or elements,
in which embodiment, or in insubstantial variations thereof, only
the features or elements specifically stated or described are
present.
* * * * *